1. Field of the Invention
The present invention relates to a turbine engine, and particularly to such an engine having a multi-stage compressor with engine support struts in the inlet of the compressor.
2. Description of the Prior Art
It is known that compression ratios in turbine engines being presently made are very high and that compressors in turbine engines are designed to perform best for cruising performance. As a result, there is a greater chance of instability occurring which might result in engine surge at off design modes of operation such as at starting and idling.
In order to overcome such problems, many turbine compressors have provisions for bleeding off air interstage, thereby increasing the stable operating range of the compressor. This bleed air, however, which is normally discharged overboard, contains energy which, of course, is lost. A typical bleed air interstage compressor with a bleed valve is described in U.S. Pat. No 3,360,189, Cook, issued Dec. 26, 1967.
Attempts have also been made to provide pre-swirl to the air as it is being drawn into the compressor during low power operation. These attempts have included the provision of a variable angle intake support strut whereby the struts give a pre-swirl direction to the air as it is being drawn but the angle is reduced as the engine approaches design mode operation. However, any mechanical solution of this type to provide pre-swirl to the air drawn into the compressor renders the structure more complicated, and increases the weight thereof.
Reference is made now to U.S. Pat. No. 3,751,909, Kohler, 1973. In this patent, which refers to a multi-shaft turbo-jet engine, there is mention of bleeding air from the outlet of a compressor stage which passes through a turbine outlet strut and back through a stator vane preceding the compressor stage in question. The apparatus described in the U.S. patent is complex and can relate only to a multi-shaft, multi-compressor turbine engine. The apparatus in Kohler is also used for cooling the hot turbine gas struts by way of drawing air between two compressors and having it pass through the engine strut beyond the turbine. The apparatus in Kohler makes provisions to take air from the exit area of the compressor which is normally not desirable since it represents an unnecessary loss of higher energy air.